In the forthcoming evolution of the mobile cellular standards like Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) will occur. A proposal for such a new flexible cellular system is Third Generation (3G) Long Term Evolution (3G LTE) that can be seen as an evolution of the 3G WCDMA standard. Such a system is described in e.g. TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, 3GPP, Release 8. This system will use OFDM as multiple access technique (called OFDMA) in the downlink.
A Mobile terminal supporting 3G LTE Release 8 is required to have two receive antennas, as well as is required to support bandwidths between 1.4 and 20 MHz. In general an OFDM receiver with two receive antennas consist of two front end receivers with analog radios and analog-to-digital converters, and a baseband processor including (among other things) two Fast Fourier Transforms (FFT), channel estimation blocks for all channel paths and a channel demodulation block. The complexity of these baseband blocks scales linearly with bandwidth.
One of the major driving factors for the cost of a mobile platform is the baseband chip area. This is especially true for low cost/high volume terminals, maybe not supporting the highest LTE data rates. Further, for high capacity OFDM system a large part of the chip area is memory where intermediate results are stored. An example is sub-carrier data that must be stored for the demodulation until the channel estimation has been completed. As a practical example; assume a 20 MHz 3GPP LTE system and a mobile terminal with 2 receive antennas and 1200 sub-carriers and a delay in the channel estimator of 7 symbols. This means that we have to store 16800 complex values, where each complex value may take 2*8 bits, which summarize to approximately 150 Kbits of memory. For more advanced setups these baseband parts need to have even more memory. For comparison, the total baseband memory (incl. memory for HARQ (Hybrid Automatic Repeat Request), etc) is 2 times that size in the 20 MHz case and hence the channel estimation part above consists of a significantly large part of the total memory needed.
Furthermore, the channel estimation part scales with bandwidth. Hence a system bandwidth of 20 MHz requires approximately twice the memory compared to a system bandwidth of 10 MHz. However, looking at the most likely LTE deployments, bandwidths above 10 MHz will likely become quite rare and system and the high volume of LTE modems will be in the 3-10 MHz system case. If low cost terminals uses prior art receivers designed for supporting optimum performance also for the rare 20 MHz bandwidth, the overall chip area (cost) might be too large, reducing the margin per platform.
Thus there is a need for receivers still fulfilling the LTE 20 MHz requirements, but optimized for low cost and optimized performance for lower bandwidths.
Therefore, it is an object of embodiments of the invention to provide a method in which requirements for a high system bandwidth can be fulfilled with a reduced memory capacity.